1. Field of the Invention
The present invention relates to an insulating resin composition and/or a transparent insulating resin composition, and more particularly, it relates to a resin composition suitable for screen printing to use in a thin-film type electronic device, as well as to a resin composition for constituting an insulating film for use in a thin-film electronic device.
The present invention also relates to a solar cell comprising the resin composition above.
2. Description of the Prior Art
In a thin-film type electronic device (i.e., an electronic device constructed by principally stacking thin films) such as a thin-film solar cell, various types of resin composition have been studied for forming an insulating film for use as an interlayer dielectric film in a multilayered structure or an insulating film in a multi-level crossed interconnection.
The resin composition above should possess insulating properties for use as an interlayer insulating film, adhesiveness to adhere the upper and the lower layers, weathering resistance, moistures resistance, heat resistance, wear resistance, resistance against scratches, flexibility, surface hardness, coating applicability of an ink by means of screen printing and the like, and hardenability.
Studies have been made heretofore on thermosetting, thermoplastic, or ultraviolet (UV) curable resin compositions such as styrene resins, saturated polyester resins, unsaturated polyester resins, epoxy resins, alkyd resins, silicone resins, acrylic resins, and fluororesins. Details on the studies as described in, for example, JP-A-61-218625 (the term xe2x80x9cJP-A-xe2x80x9d as referred herein signifies an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d).
However, in spite of the proposed resin composition above, a resin composition which satisfies the requirements with good balance is yet to be developed.
Particularly, in a multilayered structure comprising stacked thin films such as that in a solar cell or a like thin-film type electronic device with an interlayer insulating film or an insulating film for multi-level crossed interconnections, none of the known resin compositions was found to satisfy the required characteristics enumerated above as well as other characteristics concerning printability of an ink, the hardenability of the printed film, and productivity. Accordingly, no resin composition developed heretofore is suitable for use as an interlayer insulating film or an insulating film for multi-level crossed interconnections.
Furthermore, an extensive study has been made of the resin encapsulant for thin-film electronic devices, especially the resin composition that forms a transparent protective film on the surface of a solar cell of thin-film structure.
The resin composition for such use needs not only transparency but also many characteristic properties required for protective film (such as weather resistance, moisture resistance, heat resistance, wear resistance, scratch resistance, flex resistance, surface hardness, and adhesion to substrate and thin film). It also needs a good printability when used in the form of ink for screen printing.
Conventional resin components for transparent protective film are formed from a variety of thermoplastic and thermosetting resins and UV-curing resins, such as styrene resin, saturated polyester resin, unsaturated polyester resin, epoxy resin, silicone resin, acrylic resin, and fluoroplastics. Their detailed descriptions will be found in Japanese Patent Publication No. 173342/1988 and Japanese Patent Laid-Open Nos. 69874/1981 and 218625/1986.
However, none of the conventional resin compositions meet requirements for the above-mentioned characteristic properties. Especially when used for transparent protective film on solar cells of thin film structure, they are not satisfactory in transparency, properties required of protective film, printability of ink, curability, and productivity.
An object of the present invention is to provide a resin composition for forming an insulating film or a transparent protective film suitable for a thin-film type electronic device structure, and particularly, to provide a resin composition for fabricating an insulating film suitable for an amorphous silicon solar cell utilizing a flexible resin substrate, yet, with high productivity and utilizing an inexpensive and lightweight base material.
An object of the present invention is, more specifically, to accomplish the improvements below by overcoming the problems.
(1) Improving moisture resistance and water resistance of the insulating film
An object of the present invention is to improve the moisture resistance and water resistance of the resin composition constituting an insulating film, thereby improving weathering resistance of the resin composition.
The transparent resin composition should have good resistance to deterioration by moisture, adequate ability to prevent the penetration of moisture into the inside covered by the transparent protective film comprising the transparent resin composition, and high weathering resistance.
Conventional resin compositions suffered changes in composition due to the reaction with the moisture or water of the surroundings. As a result, due to the deterioration of the amorphous silicon phase or to the denaturation of the electrode material, a solar cell using the resin was found to lose the photoelectric conversion effect and to deteriorate the electric properties with passage of time.
(2) Improving heat resistance and surface hardness of the insulating film
Another object of the present invention is to increase the heat resistance and the hardness of an insulating film comprising the resin composition. More specifically, in the fabrication of a thin-film electronic device such as a solar cell, a flexible printed circuit (FPC) is provided or a lead wire is adhered to the electrode portion that is provided to externally take out the electric power by applying heat under pressure at a temperature of 100xc2x0 C. or higher for soldering or heat sealing. At the same time, the interlayer insulating film and the multi-level crossed insulating films or an insulator around electrodes are also subjected to heat shock or thermal welding.
The thermal deformation of the insulating films due to heat shock or thermal wedding can be prevented from occurring by increasing the hardness and the heat resistance of the insulating films or transparent protective films. Furthermore, insulating failure and defective appearance can be avoided.
Moreover, the heat resistance of the interlayer insulating film and the multi-level insulation film can be further improved in a device in which heat accumulation occurs during the operation of the device, or in a device which is exposed to a heat at high temperature, such as a car-use solar cells and the like.
Furthermore, in case of forming an ITO (indium tin oxide) transparent electrode, the insulating properties can be maintained by preventing physical or chemical damage from occurring on the printing film and thereby increasing heat resistance of the film.
Vehicle-mounted solar cells, which are subject to high temperatures, are protected from failure and surface scratches, which reduce light transmission and hence conversion efficiency, if they are covered with a protective film having improved hardness and heat resistance
(3) Improving wear resistance
A still other object of the present invention is to improve the wear resistance of an insulating film comprising the resin composition.
In case of laminating thin films by means of, particularly, a roll-to-roll process (a fabrication process which comprises continuously forming the device and the like by performing each of the unit operations such as film deposition, printing, and laser processing while taking up a rolled flexible substrate with another roll), there are problems such as of scratches and the like which generate on the interlayer insulating film or the transparent surface protection film when the surfaces of the upper layer and the lower layer of the flexible substrates are rubbed against each other, or when the surfaces of the flexible substrate and the guide roll are rubbed against each other.
It is therefore necessary to maintain wear resistance to prevent, for instance, inhomogeneity from occurring on an inorganic thin film such as an amorphous silicon film, a loss in performance, and a drop in production yield due to a defective appearance.
Improvement on durability in transparency:
Conventional curable resin compositions developed so far to meet the above-mentioned requirements (1) to (3) are liable to discoloration (which decreases transparency) due to attack by heat, moisture, UV light ozone, etc. when used for outdoor solar cells. The transparent resin composition should give a protective film which retains its transparency for a long period of time.
(5) Improving adhesiveness between the upper and the lower layers
A yet other object of the present invention is to improve the adhesiveness between an insulating film comprising the resin composition and the upper and the lower layers adjacent thereto.
In a multilayered thin film device, for instance, an amorphous silicon solar cell using a low-cost flexible substrate such as a resin substrate or a metallic substrate as the base material, a flexible insulating resin composition which assures excellent adhesiveness between layers (adhesiveness between the insulating film and the adjacent upper and lower layers) is required to achieve an insulation between an upper and a lower stacked thin film (layer) under various environments which accompany bending of the thin film device.
It is also required to relax the internal stress to prevent deformation or change in size, such as curling, of the flexible solar cell comprising a stacked structure of a thin film having a large internal stress such as an amorphous silicon layer and an ITO transparent electrode layer.
(6) Improving laser processability
Another object of the present invention is to improve laser processability of an insulating film comprising the resin composition.
To implement a multilayered thin film device comprising a multi-level insulation and a multi-level interconnection established in high precision, processing such as a laser scribing or a laser bonding using a YAG laser and the like is indispensable. Thus, the resin composition constituting the insulating film must be designed as such to be successfully cut at high precision.
(7) Improving hardenability
A still other object of the present invention is to lower the thermosetting temperature of a resin composition or a transparent resin composition as possible without impairing the degree of cross linking, yet elongating the pot life in its ink state.
Concerning productivity in forming a resin composition in printing to obtain an insulating film or a transparent film, it is advantageous if the thermosetting reaction proceeds efficiently at a temperature as low as possible, and if the pot life of the ink is long.
Particularly, in case of a flexible solar cell and the like in which a plastic film having low thermal resistance is used, an insulating resin composition or a transparent resin which undergoes thermosetting at a lower temperature is important in preventing thermal deformation from occurring on a solar cell.
The present invention achieves the objects of the present invention shown representatively in (1) to (7) above.
According to an aspect of the present invention, there is provided a resin composition obtained by mixing a first component comprising a polyfunctional isocyanate compound; and a polyol based second component comprising polymers or oligomers having reactive hydroxyl groups which react with isocyanate groups to principally form urethane bonds.
In accordance with another aspect of the present invention, there is provided a resin composition as above, characterized in that the first component comprises a blocked polyfunctional isocyanate compound which releases an isocyanide group upon heating.
In accordance with a still aspect of the present invention, there is provided a resin composition as above, characterized in that the second component comprises a single system or a mixed system of a phenoxy resin or a biphenol-type epoxy resin having a number average molecular weight of 1,200 or more but 50,000 or less.
In accordance with a yet other aspect of the present invention, there is provided a resin composition as above, characterized in that the second component comprises a single system or a mixed system of a phenoxy resin or a bisphenol-type epoxy resin having a number average molecular weight of 2,500 or more but 20,000 or less.
In accordance with a further other aspect of the present invention, there is provided a resin composition as above, characterized in that an additive comprising a silicone-based or an acrylic-based polymer, or a polymer of mixed system thereof is added at a quantity of from 0.001 to 5% by weight with respect to the resin composition comprising the first and the second components.
In accordance with another aspect of the present invention, there is provided a resin composition as above, characterized in that a composite is formed by adding fine particles of at least one selected from the group consisting of SiO2, Al2O3, TiO2, CaCO3, and carbon black as fine pigment particles.
In accordance with a yet other aspect of the present invention, there is provided a solar cell comprising a resin composition above.
The present invention provides a highly reliable flexible resin composition particularly useful as an interlayer insulating film. The resin composition according to the present invention is obtained by forming, as represented by phenoxy resin, a polyol of an insulating polymer or oligomer which is resistant against hydrolysis and characterized in that it comprises a bisphenol skeleton with a rigid aromatic ring and an ether bond for rendering the structure flexible as the principal chain, said principal chain provided with reactive hydroxyl groups at high concentration; preparing a resin lacquer containing a polyurethane bond obtained by reacting the resulting polyol with a polyfunctional isocyanate or a block compound thereof in a stoichiometric quantity or in a quantity slightly in excess than stoichiometry; and obtaining a resin composition by dispersing together with a dispersing agent into the resulting lacquer, a thixotropic imparting agent such as a fine-grained SiO2 (aerosil), a dye, and a laser radiation absorbing agent such as carbon black having high electric resistivity.
The surface tension of the resin composition in the form of a paste before hardening is controlled to a value of 40 xcexcN/cm (at 20xc2x0 C., as measured according to ASTM D971 by means of platinum ring method using Dynometer manufactured by Bicchemie Co., Ltd.) or lower by adding a defoaming agent, a leveling agent, etc.
In accordance with the present invention, an adduct, a biuret, an isocyanurate (a trimer), etc., of an isocyanate monomer and trimethylol propane (TMP) is effective as a polyfunctional isocyanate compound for use as a first component.
As isocyanate monomers most generally known as the aromatic compound, there can be mentioned toluene diisocyanate (TDI), 4,4xe2x80x2-diphenylmethane diisocyanate (MDI), or xylene diisocyanate (XDI). An adduct, a biuret, an isocyanurate (a trimer), etc., of the monomers above can be used in the present invention as an aromatic polyfunctional isocyanate.
However, an aromatic polyfunctional isocyanate is higher in reactivity as compared with an aliphatic (non-aromatic) compound. It can be expected to completely harden at a lower temperature, however, the pot life thereof is very short when used as an ink, a paste as a solder resist, or a two-liquid type product by mixing it with a second component polyol. It therefore has difficulties in handling.
Other isocyanate monomer free of aliphatic groups (having no aromatic rings) include hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), hydrogenated MDI (H12MDI), and hydrogenated XDI. The aliphatic (non-aromatic) polyfunctional isocyanates of the adduct, biuret, isocyanurate, etc., of TMP with the monomers above are low in reactivity as compared with an aromatic one, however, are advantageous in increasing device reliability against environment, such as the resistances to light, weathering, etc. Accordingly, they are suitable for uses where resistance against yellowing is required, or where transparency or translucency is necessary.
Although the aliphatic isocyanate above is reacts more sluggishly as compared with an aromatic one, a longer pot life can be obtained when used as an ink (an overcoating agent) or a paste as a solder resist and the like. This ease in handling in production steps is another merit of the aliphatic product.
In accordance with another aspect of the present invention, a transparent resin composition which comprises two components mixed therein, the first one being a transparent polyfunctional isocyanate compound having no aromatic rings and the second one being a transparent polyol, in the form of polymer or oligomer, having reactive hydroxyl groups to form urethane linkages upon reaction with isocyanate groups.
Also, in the above transparent resin composition, the first component is a blocked polyfunctional isocyanate compound having no aromatic rings, which liberates isocyanate groups upon heating.
Also, in the above transparent resin composition, the second component is a phenoxy resin or a bisphenol-type epoxy resin or a mixture thereof which has a number-average molecular weight in the range of 1200 to 50000.
Also, in the above transparent resin composition, wherein the second component is a phenoxy resin or a bisphenol-type epoxy resin or a mixture thereof which has a number-average molecular weight in the range of 2500 to 20000.
Also, in the above transparent resin composition, it further comprises an additive in an amount of 0.001-5 wt % which is a silicone polymer or an acrylic polymer or a mixture thereof.
Also, the transparent resin composition in accordance with the present invention is particularly advantageous for solar cells.
The present invention is designed to form a transparent protective film from a transparent resin composition which is composed of a polyfunctional isocyanate compound and a polyol. The isocyanate compound is one which has no conjugated bonds and is or is not blocked. The polyol is either a polymer or an oligomer typified by a high-molecular weight phenoxy resin having reactive hydroxyl groups. It is transparent and resistant to hydrolysis and has rigid main chains. Reaction between the isocyanate compound and the polyol (with hydroxyl groups in a stoichiometric amount or in an excess amount) give rise to a product having polyurethane linkages which serves as the protective film.
Further, the surface tension of the paste used during the film application is controlled to be 40 xcexcN/cm or less (20xc2x0 C.) (platinum ring method using dynometer produced by BYK-Chemie GmbH company under the conditions of ASTM D971) by additive agent such as antifoaming agent or leveling agent.
According to the present invention, the first component of the transparent resin composition may be a polyfunctional isocyanate compound having no aromatic rings. It includes diisocyanate, such as hexamethylenediisocyanate (HDI), and triisocyanate. Examples of the latter include an adduct of trimethylolpropane with three HDI molecules, a trimer of HDI joined by urea linkages (or biuret linkages), and a trimer of HDI forming an isocyanurate. (The last one yields a polyurethane resin superior in weather resistance and heat resistance.) Additional examples of the isocyanate compound having no aromatic rings include isophoronediisocyanate (IPDI). This isocyanate has weather-resistance and heat-resistance as in HDI, and is effective in obtaining a hard polyurethane resin, and has a long pot life when mixed with a polyol.
This IPDI forms an adduct with TMP as in HDI, and forms an isocyanurate as a trimer, and is effective in obtaining an excellent polyurethane resin as a trifunctional isocyanate.
Hydrogen added diphenylmethandiisocyanate (H12MDI) is effective as an isocyanate having no aromatic ring, and a polyfunctioned isocyante can be used.
Preferred representative examples for the block polyfunctional isocyanate compound (block isocyanate) for use as the first component of the above insulating resin component or transparent resin component contain an active hydrogen. Principal example is a compound (which releases free isocyanate groups by heating at a temperature of from 160 to 180xc2x0 C. for a duration of 30 minutes) comprising isocyanate groups blocked with hydroxyl groups originated from a block agent such as phenol, cresol, and isononyl phenol. By mixing it with a polyol in such a manner that free isocyanate be in an equivalent quantity with the hydroxyl groups in polyol, a one-liquid type product can be obtained needless of taking the pot life into consideration.
A curing catalyst such as dibutyltin dilaurate or a dissociation catalyst such as triethylenediamine (DABCO) may be added in a small quantity in preparing the resin.
An oxime such as methyl ethyl ketoxime can be mentioned as another example of a block agent (which dissociates by heating at about 140xc2x0 C. for a duration of 30 minutes).
A block agent comprising a lactam group such as xcex5caprolactam, or such comprising an amino group, an amide group, or an imide group can be used as well (these thermally dissociate by heating at about 160xc2x0 C. for a duration of 30 minutes).
Furthermore, a dicarbonyl compound such as diethyl malonate or ethyl acetoacetate is also effective as a block agent. Because a block agent of this type dissociates the isocyanate groups at a lower temperature (i.e., by heating at about 100xc2x0 C. for 30 minutes), it can be used advantageously as a one-liquid product having an intensified thermosetting property.
The polyfunctional isocyanide compounds described above functions effectively as the isocyanate compounds to be combined with the block agents above. An aromatic isocyanate can be used, because no urethane reaction occurs at an ordinary temperature in case it is mixed with a polyol of the second component. Accordingly, the resulting product can be used as a one-liquid type pot life-free ink (overcoat agent), a solder resist, etc.
Furthermore, among the isocyanate compounds to be combined with these blocking agents, the above-described polyfunctional isocyanate compound having no aromatic ring is practically preferable to keep transparency.
Polyol for use as the second component comprises a single system or a mixed system of a phenoxy resin or a bisphenol-type epoxy resin having a number average molecular weight of 1,200 or more but 50,000 or less, and preferably, 2,500 or more but 20,000 or less.
In the second component, when a material of a low degree of polymerization such those having a number average molecular weight 1200 or less, the proportion of the reactive hydroxyl group decreases and the crosslinking density of the urethane bonding between the first and second component decreases.
As a result, the obtained resin component can not sufficiently exhibit the strong adhesivity with an underlying surface or the strong wear resistance or flexibility which are caused by urethan bond radicals. Also, there is a tendency that the printability of the resin component as an ink for screen printing decreases since the viscosity and elastic modululs decreases.
In particular, the viscosity and the elastic modulus decreases in the case of a transparent resin component which is not added with a pigment, making it more difficult to be printed.
Accordingly, it is preferable that the second component has its number average molecular weight not lower than 1200, more preferable, not lower than 2500.
For example, it is effective to use a material having a bisphenol A-type epoxy structure unit of which degree of polymerization is not lower than 4 and having four or more of reactive hydroxyl groups in each of which one principal molecular principal change is arranged in a pendent form. In this case, the molecular weight is about 1350.
Also, if the number average molecular weight of the second exceeds 50000, the kinds of solvents that can be used are limited. Even if it can be solved, the viscosity of the solution tends to be high and the flowability is low. As a result, it is difficult to make an ink for screen printing is difficult.
For example, when a screen printing is used with an ink of which main component is a material having a bisphenol A-type epoxy structure unit having a high degree of polymerization and having a molecular weight exceeding 50000, it is difficult for the ink to pass through the mesh of a screen having a desired fine pattern.
Also, because of the high viscosity, there is no defoaming effect with respect to enclosed bubbles when using a silicon containing defoaming agent as an additive and because it exhibits a strong consistency, it is almost impossible to uniformly mix such a second component and a polyfunctional isocyanate having no aromatic ring as a first component, resulting in that it is extremely difficult to form an urethane resin component having urethane bonds uniformly arranged.
Accordingly, in the case where using the second component having a high degree of polymerization alone, it is effective if the number average molecular weight is not higher than 50000, preferably not higher than 20000 in order to effectively utilize the characteristics of the urethane resins in view of the printability as a screen printing ink and the ease of handling.
Also, in the case where mixed resins having different molecular weights are used as a second component, it is desirable for the same reasons that the number average molecular weight is in the range of 1200 to 50000, more preferably, not lower than 2500 and not higher than 20000.
For example, phenoxy resins produced by Union Carbide Corp., such as PKHC (having a number average molecular weight of about 14,000), PKHH (having a number average molecular weight of about 15,400), or PKHJ (having a number average molecular weight of about 18,700), and Phenoade YP=50, a phenoxy resin produced by Tohto Kasei K. K may be used.
To control the suitability for printing an ink or forming a film with an ink, a bisphenol-type epoxy resin produced by Shell Kagaku K. K., such as epikote 1007 (having a number average molecular weight of about 2,900) or epikote 1009 (having a number average molecular weight of about 3,750), can be used effectively either singly or as a mixture with a phenoxy resin.
In case of utilizing the resin composition having a polyurethane bond as above as, for example, an interlayer insulating film or a transparent protective film in a flexible solar cell by uniformly forming the desired pattern by screen printing, the defoaming properties and the leveling properties of the ink (paste), as well as the wetting properties with respect to the underlying amorphous silicon film, ITO electrically conductive film, transparent insulating film, etc., must be improved. Preventing pinholes from generating and avoiding repelling of ink, as well as improving the suitability to re-coating are also important factors for realizing a higher uniformity and decreasing refuse rate of the solar cell.
As an effective means for satisfying the aforementioned requirements for a resin composition, from 0.001 to 5% by weight with respect to the resin components (the first and the second components) of a silicone based, acrylic based, or vinyl ester based additive is added if necessary to more favorably maintain screen printability of the resin ink.
Concerning a silicone based additive, for instance, it is effective to mix a trace quantity of a methylalkyl polysiloxane type silicon compound which is partially polyether-modified, an alkyl-modified compound, or a polyester-modified dimethyl polysiloxane.
Particularly, in case of a silicone based deforming agent or a leveling agent, an alkyl-modified compound of methylalkyl polysiloxane as shown in chemical formula (I) below was found favorable. 
Not only silicone compounds, but also acrylic or polyvinyl ether based oligomers or polymers (having a number average molecular weight of about 20,000 or less) added in a small quantity were found effective. In case of uniformly coating an insulating ink (e.g., a solder resist, overcoat agent, etc.) at high precision according to the desired pattern by means of screen printing, it is especially important to control the fluid properties of the ink, and particularly the thixotropy. Favorable results can be obtained by uniformly dispersing particularly fine particles of a pigment in the resin composition.
Effective fine-grained pigments include SiO2 (for instance, Aerosil produced by Degussa Corp.), Al2O3, TiO2 and high resistivity carbon black. Particularly favorable are the fine particles having a primary particle diameter in a range of from about 5 to 30 nm.
Among the fine-grained pigments above, carbon black absorbs laser radiation. Accordingly, carbon black is also effective in improving suitability of the resin to laser scribing or laser cutting.
The point in preparing an ink (paste) is to sufficiently disperse the fine-grained pigment in the resin component provided for preparing an ink (particularly, the second component). The addition of a polymer based dispersant which less transfers to the printed film surface was found effective.
Examples of the dispersant include polymer dispersants soluble to the second and the first components of the resin composition, and having, on the terminal of the polymer (i.e., an acrylic resin or a polyester resin), a proton-donating groups such as xe2x80x94COOH, xe2x80x94SO3, xe2x80x94P(O), (OH)2, or xe2x80x94OH; a proton-accepting group such as the salts of the proton-donating groups enumerated above, xe2x80x94NH2, xe2x80x94NHR, xe2x80x94NR, or R2, and furthermore a polar group having an ion pair bond such as a quaternary ammonium group.
The fine-grained pigment is preferably added at a concentration of from 3 to 20% by weight with respect to the total of the resin composition components (the total of first component, second component, fine-grained pigment, and solvent). The dispersant is functions effectively by adding it at a concentration of from 1 to 5% by weight with respect to the total of the resin composition components.